Radio frequency identification (RFID) readers are commonly used to determine the presence or absence of an asset having an RFID tag affixed thereto. Typically, an RFID reader powers an antenna, which emits an RF signal and thereby creates an RF field throughout the signal range of the antenna. An RFID tag within this RF field receives the RF signal emitted by the antenna and returns its own signal in response. The signal emitted by the RFID tag is, in turn, received by the antenna to allow the RFID reader to confirm the presence of the RFID tag and, correspondingly, the asset affixed thereto.
However, this system does not provide an indication of the physical position of the RFID tag. In order to localize the asset, or determine its relative position in relation to the RFID reader, multiple antennas have been used to create a plurality of overlapping RF fields. This increases the ability to localize RFID tags, but introduces a number of new problems.
First, the use of multiple antennas often creates misreads, which occur when an RFID tag does not receive a sufficiently strong RF signal to return a responsive signal of its own. This is because, RFID tags, particularly passive RFID tags, which utilize the RF signal transmitted by antennas for power, will not emit a responsive signal unless the RFID tags receive a sufficiently powerful RF signal from the antennas. Thus, even though an RFID tag is located within the RF field created by an antenna. The RFID tag may not respond to the received RF signal, because the received RF signal lacks the power required to elicit a response from the RFID tag. Therefore, the RFID reader registers the RFID tag as not being present, even though the RFID tag is, in fact, within the RF field of the antenna.
Second, the use of multiple antennas often creates cross-reads, which occur when the RF fields of multiple antennas interfere with each other and telegraph the RF signals sent by RFID readers and emitted from RFID tags into adjacent RF fields. Thus, a responding signal sent by an RFID tag may be received by two different antennas, thereby causing a reader to determine that the same RFID tag is located in two different positions. As such, it is often difficult to accurately determine the location of the RFID tag when multiple antennas are used.